AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency

[John Smith I]

...
This will not happen in a properly designed transmitter. It is not a
characteristic of AM.

Listen to a "strong--pure am signal" on an fm receiver, turn up the
volume on the fm receiver, something is responsible for that ... repeat
experiment with the reverse ... "imperfect world theory" proof!

In new equip (I started out decades ago, remember) voltage regulation,
filters, suppressors have much improved ... digital processing is king
and allows what analog never could achieve ...

Regards,
JS

 

[Don Bowey]

Listen to a "strong--pure am signal" on an fm receiver, turn up the
volume on the fm receiver, something is responsible for that ... repeat
experiment with the reverse ... "imperfect world theory" proof!

You are hearing the effects of the sidebands, not the Carrier.

In new equip (I started out decades ago, remember) voltage regulation,
filters, suppressors have much improved ... digital processing is king
and allows what analog never could achieve ...

Regards,
JS

In a properly designed transmitter the Carrier amplitude does not change
with modulation. I have better tools than FM receivers to prove that fact
and theory agree for AM.

 

[Jeff Liebermann]

Exactly. The modulator signal modulates the carrier wave. If there is
no modulator signal, then the carrier does not vary by amplitude or by
anything.

Brilliant. Yes, if there is no signal input, there's no change in
output.

Incidentally, in an AM system, the carrier does NOT change. You can
see that on a spectrum analyzer. Modulate all you want and the
carrier stays put at 50% of the total power output. The rest of the
power is split between the upper and lower side bands. If there is no
modulation input, then the side bands disappear, but the carrier just
stays there.

As someone mentioned, there is usually some residual FM on the carrier
usually caused by sloppy power supply regulation. Also, some
synthesizer noise. A well designed AM broadcast transmitter doesn't
have much of this junk present. The problem is that the FM that
appears on the carrier also appears on all the side bands. It doesn't
hurt if the carrier has a little residual FM, but any such junk on the
sidebands will result in a substantial increase in audible noise by
mixing with the audio.

One poster stated that the signal with the higher-frequency is
automatically the carrier wave while the signal with the lower-
frequency is automatically the modulator wave.

That was me.

This is not true.

Prove it. I explained how it works and why quite adequately. I
didn't even need to resort to formulas and calculations. The
multiplier (mixer) modulator inputs are symmetrical and identical.
Therefore the inputs are also symmetrical and indistinguishable. I
also provided a simple audio test you can do in your spare time to
demonstrate how it works.

Now, convince me that the multiplier (mixer) waveform would be
different depending on which input was the carrier or modulation.

What
I was trying to say is that an AM radio carrier wave cannot vary
significantly by anything other than its amplitude [though, as one
poster pointed out, the AM carrier can experience extremely-negligible
variations in frequency]. If an AM radio signal has that restriction,
it is the carrier wave. If an AM radio signal does not have that
restriction, then it is the modulator wave. This is true, even if the
AM carrier wave is of a lower-frequency than the modulator wave.
That's what I was trying to say.

I give up. What you've done is created a word salad. That's where
you have a mess of buzzwords, shredded together, mixed with some
window dressing, and served in a manner to imply that you have a clue
what you're disgorging. Even the most basic concepts are not sinking
in. You've also ignored multiple suggestions to read some very fine
sources on how RF and modulation works. Open book, insert face, and
come back when you have a clue as to the basics.

In AM radio, determining which is the carrier wave and which is the
modulator wave is not by which has the higher frequency but rather by
which has the restriction that I stated.

Wrong. With AM it's easy. The higher frequency is always the
carrier. Can you give me a diagram or a commonly used communications
system where the reverse might be true? I can't.

If there is no modulator signal, then no carrier signal of any type
[AM, FM, etc.] will vary by any quality [frequency, amplitude, phase,
etc.]

Yawn...

 

[John Smith I]

...
You are hearing the effects of the sidebands, not the Carrier.

DUH! And, you only have the sidebands as a result of the
carrier/modulation ...

In a properly designed transmitter the Carrier amplitude does not change
with modulation. I have better tools than FM receivers to prove that fact
and theory agree for AM.

And the time to argue the insignificant ... sharpen that razor blade,
you can then successfully split much narrower hairs ...

JS

 

[Radium]

Use simple logic, you can modulate a dc (0 Hz) with higher freq (voice),
(hint, your telephone line is an example) right?

The telephone does not use either AM or FM. It is simply the
electrical equivalent of the sound that gets into the microphone. You
input a 1 KHz tone into the microphone, telephone lines will carry a 1
KHz AC current to the destination. The louder the sound into the
microphone, the stronger the amperage in the telephone lines.

 

[Don Bowey]

DUH! And, you only have the sidebands as a result of the
carrier/modulation ...

So what? You implied or inferred that what was heard from the FM radio was
caused by the AM Carrier. Leave out the Carrier and you will hear the same
thing.

And the time to argue the insignificant ... sharpen that razor blade,
you can then successfully split much narrower hairs ...

It is important that we not confuse a person new to electronics by the type
of inane points you make. It doesn't matter a whit if someone's AM
transmitter Carrier shifts on power peaks due to poor regulation. It has
nothing to do with "AM" and everything to do with poor design.

Side issues don't help the new folks.

 

[Don Bowey]

The telephone does not use either AM or FM. It is simply the
electrical equivalent of the sound that gets into the microphone. You
input a 1 KHz tone into the microphone, telephone lines will carry a 1
KHz AC current to the destination. The louder the sound into the
microphone, the stronger the amperage in the telephone lines.

Good for you for catching that one. The effect of microphone current has
noting at all to do with AM.

 

[DTC]

ELF communications are carried out at very slow data rates, only a few
characters per hour at best.

Actually its on the order of several characters per minute using a 64
character "alphabet".

It is possible to communicate at a base band frequency of 0Hz. This is what
happens when you talk down a hard wired telephone or intercom. At a
telephone exchange (switching centre), the signals from each line are
modulated onto a higher frequency for onward transmission down a trunk wire
cable or fibre optic cable. The multiplexed high frequency modulated signals
are down converted back to audio frequencies once they reach the intended
destination.

In the old T carrier (before 24 channel digital T1) carrier, each telephone
conversation was modulated onto a low frequency radio frequency AM signal
ranging from (and don't quote me as its been over thirty years since I
worked T spans) 50 KC to 200 KC. Very similar in principle to the 5 kc wide
AM radio station signals on the 530 kHz to 1700 kHz AM broadcast band.

 

[DTC]

You
input a 1 KHz tone into the microphone, telephone lines will carry a 1
KHz AC current to the destination. The louder the sound into the
microphone, the stronger the amperage in the telephone lines.

On a side note, its actually voltage modulation towards the subscribe and
current modulation back to the central office. The earpiece is a high
impedance (2,000 ohm) device that responds to voltage variations. The
carbon microphone element 220 to 200 ohms modulates the talk battery current.

 

[kev]

Argh, that brings back fond nightmares of Ma Bell. 4Hz per voice
channel with FDM (frequency division mux). Most were FM systems, but
there were some AM implimentations (to avoid patent infringement).
Later, there were SSB systems that doubled the number of channels.

No voice Spectrum BW
channels KHz kHz AT&T ITU-T
12 60-108 48 Group Group
60 312-552 240 Supergroup Supergroup
300 812-2044 1232 Mastergroup
600 564-3084 2520 Mastergroup
3600 564-17548 16984 Jumbogroup

That does bring back memories. I worked on STC built systems that used
AM modulation using Double-Balanced Modulators and depending on "Group"
classification used either the lower or upper sideband. Up to the 60
"voice" Ch's, we had some low Baud rate Modems on as well with the
signalling frequency disabled, the spectrum usage was the the same
however the next step up was 16 Supergroups using 60-4028KHz with
Supergroup 2 not being translated. We also had a 30 Ch PCM link which
worked very well apart from the "Regenerators" being susceptible to
lightning.

 

[Ian Jackson]

The fundamental answer is no, it is not possible to generate AM where
the baseband signal is a pure 20 kHz sinewave and Fc<20kHz. The reason
is that the modulated waveform consists of the sum of a sinewave at Fc,
a sinewave at Fc+20kHz, and a sinewave at Fc-20kHz. If Fc<20kHz then
one of the components becomes a "negative" frequency. So the carrier
must be greater than the baseband signal to prevent this.

I'm afraid that this is not correct. The 'laws of physics' don't
suddenly stop working if the carrier is lower than the modulating
frequency. However, there's no need to get into complicated mathematics
to illustrate this. Here is a simple example:

(a) If you modulate a 10MHz carrier with a 1MHz signal, you will produce
two new signals (the sidebands) at the difference frequency of 10 minus
1 = 9MHz, and the sum frequency of 10 plus 1 = 11MHz. So you have the
original carrier at 10MHz, and sideband signals at 9 and 11MHz (with a
balanced modulator - no carrier - only 9 and 11MHz).

(b) If you modulate a 1MHz carrier with a 10MHz signal, you will produce
two new signals (the sidebands) at the difference frequency of 1 minus
10 = minus 9MHz, and the sum frequency of 1 plus 10 = 11MHz. The
implication of the negative 'minus 9' MHz signal is that the phase of
the 9MHz signal is inverted, ie 180 degrees out-of-phase from 9MHz
produced in (a). So you have the original carrier at 1MHz, and sidebands
at 9 and 11MHz (again, with a balanced modulator - no carrier - only 9
and 11MHz).

The waveforms of the full composite AM signals of (a) and (b) will look
quite different. The carriers are at different frequencies, and the
phase of the 9MHz signal is inverted. However, with a double-balanced
modulator, you will only have the 9 and 11MHz signal so, surprisingly,
the resulting signals of (a) and (b) will look the same.

[Note that, in practice, many double-balanced modulators/mixers put
loads of unwanted signals - mainly due the effects of harmonic mixing.
However, the basic 'laws of physics' still apply.]

Finally, although I have spoken with great authority, when I get a
chance I WILL be doing at test with a tobacco-tin double-balanced mixer,
a couple of signal generators and a spectrum analyser - just to make
sure that I'm not talking rubbish. In the meantime, I'm sure that some
will correct me if I'm wrong.

Ian.
--

 

[DTC]

Argh, that brings back fond nightmares of Ma Bell.

And of splicing damaged buried plant in a wet trench...that stuff had a
bite to it.

 

[Don Bowey]

Actually its on the order of several characters per minute using a 64
character "alphabet".


In the old T carrier (before 24 channel digital T1) carrier, each telephone
conversation was modulated onto a low frequency radio frequency AM signal
ranging from (and don't quote me as its been over thirty years since I
worked T spans) 50 KC to 200 KC. Very similar in principle to the 5 kc wide
AM radio station signals on the 530 kHz to 1700 kHz AM broadcast band.

The O Carrier systems went from a low of about 32 kHz up to 164 kHz if I
remember right. And the mainstay of long-haul communications (L Carrier)
channel bank, was 64 - 108 kHz.

One of the most strange Carrier Systems I worked with was a 1930s vintage H
Carrier, one channel ssb "system" operating at about 12 kHz, and it ran
without automatic synchronization. That was in the 60s. We used it as a
maintenance channel in a voice over data configuration for a gap-filler
radar site. I've never seen a more extreme merging of old and new
technologies.

Don

 

[Don Bowey]

Radium wrote:
...

You miss the simple point, the dc is the carrier ... instead of dc, you
could put a 1 hz signal on the line and modulate it with your voice,
indeed, you can put a 30 hz signal on the line and modulate it with your
voice--if you can tollerate a bad 30 hz hum! But, who knows, perhaps
you are tone deaf to the 30 hz hum and would like it ...

JS

But you miss the basic point......

The topic was Amplitude Modulation.

 

[Don Bowey]

You are an idiot ... bother some one who has the time to take you to
task ...

Open your mind.

 

[[email protected]]

Radium:

Use simple logic, you can modulate a dc (0 Hz) with higher freq (voice),
(hint, your telephone line is an example) right?

However, when you get into RF--possible, usable, desirable are seperate
and distinct things.

Again, with simple logic, modulating a 30 CPS signal with limited voice
freq (say 5K wide) is going to create a LOT of harmonics and mixed
signals, ain't it? Suggesting a very wide band receiver would be needed
to begin with ... in my humble opinion, and for various reasons, NO, it
is NOT possible ...

Regards,
JS

radium, I applaud you in your interest generating discussion.

if there are no questions there are no answers; dumb or smart!

to me your questions came across wonderfully, and generated both
responses.

as humans we stand on two legs, most of us that is.

men get the honor and privilage to stand on three legs from time to
time.

this is our blessing and our curse!

ps. how would u like to change the cell phone industry? and your
discussion group of course!

remeber all things are possible!!!!!!!!!!!!!!!!

 

[John Smith I]

Open your mind.

Geesh!

Hook up a 20X linear behind an xmitter and see if you can't find some
artifacts ...

JS

 

[kev]

And of splicing damaged buried plant in a wet trench...that stuff had a
bite to it.

The Coax we used for the repeaters was fed with 250-0-250V DC and the
current was regulated at 49mA. The only time the cable jointers worked
on it the power feed was disabled.

 

[Don Bowey]

Geesh!

Hook up a 20X linear behind an xmitter and see if you can't find some
artifacts ...

JS

Sheesh....

That has nothing to do with helping someone understand AM. It appears you
are more interested in dumping your blog on this board, than providing
something to clarify the real answers for an electronic novice.

 

[Don Bowey]

Geesh!

Hook up a 20X linear behind an xmitter and see if you can't find some
artifacts ...

JS

By the way, I have. A 10B to a GPT750.